If you know a religion to be false, should you tell its followers? (idea)

It is a moot point. Most religions are based on faith. Proof, evidence, and fact have little impact upon the faithful. Try providing your proof to someone of faith (the more zealous the believer the better). You'll probably find this exercise quite frustrating. Again, it's the nature of faith; belief without proof, fact, or evidence. Logic and reason are useless against faith.

So, tell them if you will. But don't be surprised when the followers choose their faith over your proof. For the record, I'm agnostic.